What are the primary causes of bradycardia in a general adult population?

Primary Causes of Bradycardia

Bradycardia in adults stems from two broad categories: reversible/extrinsic causes (medications, acute MI, electrolyte disorders, intoxication) and intrinsic cardiac conduction system disease (sinus node dysfunction and AV block). 1

Reversible/Extrinsic Causes (Must Be Identified First)

The European Society of Cardiology emphasizes that identifying potentially reversible causes is the critical first step before considering permanent interventions. 1 In a registry of 277 patients presenting to emergency departments with compromising bradycardia, the distribution of causes was:

• Adverse drug effects: 21% - Most commonly beta-blockers, calcium channel blockers, digoxin, and antiarrhythmic agents 1, 2, 3
• Acute myocardial infarction: 14% 1
• Intoxication: 6% 1
• Electrolyte disorders: 4% - Including hyperkalemia, hypomagnesemia, and hypocalcemia 1, 4

Additional extrinsic causes include:

• Hypoxemia - A common and often overlooked cause that should be assessed immediately 1
• Metabolic disturbances - Uremia, hypothyroidism, hypothermia 4
• Increased vagal tone - Situational triggers (micturition, defecation, cough, vomiting), carotid sinus hypersensitivity 1

Intrinsic Cardiac Causes (Primary Conduction System Disease)

When reversible causes are excluded, intrinsic cardiac disease accounts for approximately 49% of cases presenting with compromising bradycardia. 5

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Failure of impulse formation - The sinus node fails to generate impulses at an appropriate rate 1
• Sinoatrial conduction block - Impulses are generated but fail to exit the sinus node 6
• Brady-tachycardia syndrome - Alternating bradycardia and tachyarrhythmias, particularly atrial fibrillation (occurs in 8.2% at diagnosis, increasing to 15.8% during follow-up) 1

Atrioventricular Conduction Disturbances

The American Heart Association classifies AV blocks by severity and location: 1

• First-degree AV block - PR interval >0.20 seconds; generally benign but can cause symptoms when PR >0.3 seconds due to atrial contraction occurring too early in diastole 1
• Second-degree AV block, Mobitz Type I (Wenckebach) - Block at the AV node level; often transient and asymptomatic 1
• Second-degree AV block, Mobitz Type II - Block below the AV node in the His-Purkinje system; frequently symptomatic with potential to progress to complete heart block 1
• Third-degree (complete) AV block - No impulses conduct from atria to ventricles; can be permanent or transient depending on etiology 1

Pathophysiologic Classification Framework

The European Society of Cardiology provides a useful distinction: 1

• Persistent bradycardia clearly indicates intrinsic AV block or sinus node dysfunction
• Intermittent bradycardia has less clear meaning, often resulting from variable contributions of both intrinsic and extrinsic mechanisms (the same event may be diagnosed as either primary cardiac arrhythmia or cardio-inhibitory reflex depending on clinical context)

Critical Clinical Pitfall

The most important pitfall is failing to identify reversible causes before attributing bradycardia to intrinsic disease. 1 Among emergency presentations, over 40% of compromising bradycardia cases have reversible etiologies (drugs 21%, acute MI 14%, intoxication 6%, electrolytes 4%). 1 Permanent pacemaker implantation in these patients would be inappropriate and potentially harmful.

Age-Dependent Considerations

Bradycardia significance is highly age-dependent: 1

• A heart rate of 45 bpm may be normal in adolescents and young athletes
• The same rate indicates profound bradycardia in newborns or infants
• In elderly patients, consider decreased hepatic and renal function affecting drug metabolism 2